Will these fit the ZD Racing MT8 Pirates3?

Yes. They are listed as rear hub carriers for the MT8 Pirates3, part number 8051. Check your car's manual or parts list for 8051 before ordering.

One set of rear hub carriers. The listing does not itemize bearings, pins or screws, so reuse your existing hardware unless your kit includes it.

Are they stronger than the stock hubs?

ZD says the aluminum alloy resists large impacts without deforming. Metal hubs are stiffer than plastic, so they bend less, but they transfer more shock to the suspension arms and hinge pins.

Do I need to change anything else when fitting them?

Usually not. They mount in the stock location, but check that your hinge pins and bearings are straight and undamaged before fitting new carriers.

Are they worth it over plastic replacements?

If you keep bending or breaking stock rear hubs, aluminum holds alignment better and should last longer. If you prefer some flex to absorb crashes, plastic is the safer choice.